This invention relates to improvements in the fabrication of heat exchangers of the type wherein a series of very closely spaced fins are to be bonded to a fluid-carrying tube passing through aligned holes in the fins. One environment where heat exchangers of this type may be used is that of the viscous drag type heat exchangers utilized for the condenser and evaporator of a heat pump, such as shown in U.S. Pat. No. 3,866,668.
Two problems arise in the fabrication of such heat exchangers. The first involves provision of means for establishing uniform longitudinal spacing of the fins along the length of the tube. Particularly in a viscous drag air pumping type of rotating heat exchanger, there is an optimum spacing to produce maximum air flow and maximum heat exchanging efficiency. One method which has been employed in the past is the use of an annular flange or collar integral with the fin and surrounding punched tube-receiving hole in the fin. Such collar abuts a portion of the adjacent fin and thereby establishes a minimum spacing therebetween.
These gap-establishing collars have not been entirely successful. It is not possible to turn a sharp right angle flange when punching a hole in the fin, and the required radius of the bend establishes an annular conical depression surrounding the tube on the side of the fin away from the collar. The free end of the collar on the adjacent fin will tend to nest into this depression to a variable degree, destroying the intended close control of the minimum fin spacing.
These collar-like spacers also interfere with the successful brazing of aluminum fins and tubes. A sound braze is essential to provide optimum heat flow across the joint between the tube wall and fin. The collar interferes with proper cleaning of the assembly, such as by ultrasonic bath and vapor degreasing operations, because dirt particles can be trapped in the crevices between the collar and the tube. Also, if the tube is clad with a brazing material, such cladding does not "see" the magnesium which may be placed in the brazing oven (in powder or electrode form) for the purpose of driving out impurities in the clad. Furthermore, the rounded intersection between the collar and the tube prevents the formation of an optimum brazing fillet.
Accordingly, it is the object of this invention to provide an improved construction of the joint between the fin and tube, to achieve reliable control over the fin spacing and to provide a sound brazed joint between the fin and tube for maximum heat conductivity.